Angular scintillation reduction device in a radar and a radar containing such a device

ABSTRACT

An angular scintillation reduction device used more especially in radar equipments making deviation measurements. 
     It contains a circuit (60) receiving the video outputs of a radar receiver, the sum Σ, range D and deviation measurement ε signals respectively on three terminals (3, 28 and 2). It delivers on a terminal (30) the values of the deviation measurement signal ε corresponding to the maximum values of the sum signal Σ. 
     It is applicable to all electromagnetic detection equipments which make deviation measurements.

The present invention covers an angular scintillation reduction deviceused more especially in deviation measurement radars.

The methods for the measurement of the angular position of a target by aradar, also known as a method of deviation measurement or a monopulsemethod, are based on the comparison of two or more different diagramswith the antenna aimed at the same target. There may be one or moremeasurements, in elevation and bearing for example.

The angular measurements are determined from the amplitude of anelectric signal which represents the measurement. Apart from radarnoise, the angular measurements are upset by a scintillation error,which is mainly due to the complex character of the target that may beassimilated to the association of several elementary reflectors. Thiserror limits the performance of radar systems and in particularautomatic tracking systems.

A radar system with means for measuring the angle or angular deviationgives at least two antenna diagrams on reception.

They are used in at least two reception channels normally known as thesum channel, which delivers a sum signal Σ, and the difference channel,which delivers a difference signal Δ. The means for calculating theangular measurements generally produce the quotient ##EQU1## Thequotient called ε is a measure of the off-center angle of the target asdetected with respect to the axis of the two antenna diagrams. Filteringof the angular measurement errors may be done by means of simple linearand non-linear filters. However, the residual level of angularmeasurement error is still not negligable for these types of filter whenit is a question of scintillation.

The present invention proposes to correct this disadvantage.

The device in accordance with the invention contains at least a means,which receives from the deviation measurement radar receiver the sum Σ,difference Δ and deviation measurement ε signals, this last one beingproduced from the Σ and Δ signals, and only retains in the measurementsof the ε signal those corresponding to the maximum values of the Σsignal.

Other advantages and characteristics of the present invention willappear from the description which follows together with the figureswhich show:

in FIG. 1, the device in accordance with the invention used with a fixedgain deviation measurement radar receiver,

in FIG. 2, the device in accordance with the invention used at theoutput of a deviation measurement radar receiver with an automatic gaincontrol circuit.

A target may be reduced to a small number of reflecting surfaces in thedirection of the radar. Let M_(i) and φ_(i) be the amplitude and phaseof the wave reflected by one of these small surfaces on the target.

The sum Σ and difference Δ signals may be written:

    Σ=Σ.sub.i M.sub.i e.sup.jφ i

    Δ=Σ.sub.i M.sub.i θ.sub.i e.sup.jφ i

in which θ_(i) represents the angular deviation of the direction of areflecting surface under consideration with respect to the axis of thetwo antenna diagrams.

The corresponding deviation measurement signal is then written: ##EQU2##In the special case in which φ_(i) -φ_(K) =0, which corresponds to areflection in phase of the various surfaces of the target and hence tozero evolutions of this target with respect to the radar, the sum signalΣ is a maximum and the deviation measurement signal ε becomes: ##EQU3##In all other cases, in particular when the instantaneous signal issmall, because of the relative variations of the phases φ_(i) and φ_(K),the deviation measurement signal ε may suffer big fluctuations aroundthe radioelectric center of gravity of the target.

The device in accordance with the invention proposes to correct thisdisadvantage by selecting deviation measurements corresponding to valuesof the Σ signal greater than an increasing, predetermined threshold,thus making it possible to reduce the fluctuations in the targetdirection as seen by the radar.

FIG. 1 shows the schematic diagram of the device in accordance with theinvention which makes this selection of the values representing thedeviation measurement signal ε.

It contains a circuit 60, which is divided into a circuit 27, thatreceives the sum signal Σ and the range D of the target and calculatesthe signal υ U=Σ² /Σ² where Σ represents the mean value of the sumsignal Σ, and a circuit 20, that receives signal U and the deviationmeasurement signal ε. Circuit 27 contains a squaring circuit 1 connectedto an integrating circuit 4, which includes, for example, a resistor 21and a capacitor 22, an adder circuit 7, a divider circuit 8 and possiblya dynamic error correction circuit 6.

Circuit 20 contains two identical circuits 5 and 11 for sampling anddigital coding, a peak detection circuit 9, which includes, for example,two resistors 23 and 25, a diode 24 and a capacitor 26, a dividercircuit 10, a circuit 12, which calculates a predetermined function ffrom binary words x_(k) applied to its input, a circuit 13, whichincludes a multiplier circuit 14, an adder circuit 15, a circuit 17,which introduces a predetermined delay, two multiplier circuits 16 and18, a memory 19 and a circuit 51 which calculates 1-K_(k), where K_(k)is the input signal to this circuit.

This device operates as follows:

Signals Σ and D coming from the receiver are available on terminals 3and 28 respectively. Circuit 1 connected to terminal 3 delivers a signalΣ², which is applied to the integrator 4 of time constant τ=RC and toone of the two inputs of divider circuit 8 whose other input isconnected to the output of integrator 4 through a summation circuit 7.The output of integration circuit 4 delivers the mean value of thesignal Σ², which is noted as Σ² and is applied to one of the two inputsof adder circuit 7 and possibly to a circuit 6 which also receives theradar-target range D from terminal 28.

Circuit 6 calculates the quantity--4.Σ².(v_(r) /D).R.C. in which R and Care the value of the resistance 21 and of the capacitor 22 respectively,and v_(r) is the measurement of the estimated radial speed of the targetwhich may come from a circuit outside the device in accordance with theinvention through a terminal 70. The output of circuit 6, when appliedto the second input of adder circuit 7, delivers a corrective term whichis added algebraically to the value of signal Σ² and makes it possibleto compensate for the delay introduced by integration circuit 4. Dividercircuit 8 delivers a signal U=Σ² /Σ², which corresponds to thestandardized fluctuations of the signal Σ² and is applied to the firstinput of divider circuit 10 and peak detector circuit 9. Circuit 9, bymeans, for example, of two resistors 23 and 25, diode 24 and capacitor26, makes it possible to retain only the signal V which corresponds topeak signal U.

Divider circuit 10, which is connected to circuit 9, then delivers thesignal x=U/V to circuit 11, which samples signal x and codes digitallythe amplitude of each of the samples s_(k). The numbers x_(k) less than1 then act as a factor of merit for the values of deviation measurementsignal ε. Hence the measurement of ε will be considered as good if theassociated coefficient x_(k) is closed to 1. It is sometimes necessaryto increase the speed of variation of factor of merit x_(k) to allow abigger selection of good measurements of signal ε. For this purpose, afactor of merit defined as K_(k) =f(x_(k)) is used in preference to thevalue x_(k), f being a predetermined function. Experimental measurementshave shown that function f can be chosen preferentially, but notrestrictively, such that: K_(k) =e⁻β(1-x.sbsp.k.sup.)n in which β is apositive number, 5.5 for example, and n:3 for example.

The output of circuit 12 which calculates factor of merit K_(k) isconnected to one of the two inputs of a first multiplier circuit 14. Thesecond multiplier circuit 16 has an input circuit 51 performing thesubtraction 1=K_(k) from the output signal of the calculating circuit12. Multiplier circuit 14 receives at its second input deviationmeasurement signal ε, which appears at input terminal 2 and waspreviously sampled and coded in sampling and coding circuit 5, thiscircuit 5 converting it into a series of binary words noted ε_(k) at asampling frequency f_(c) the same as that of circuit 11. The output ofmultiplier circuit 14 then delivers a signal represented by the productε_(K).K_(k) ; if the value of index k is bad, K_(k) is near zero and thecorresponding value ε_(k) will not be taken into account. In order toallow for a certain integration time, by taking into account previouslymeasurements of signal ε_(k), the output of multiplier circuit 14 isconnected to an adder circuit 15 whose output is fed back to its secondinput through delay circuit 17 and multiplier circuit 16 whichmultiplies the input signal by the coefficient 1-K_(k).

The output of adder circuit 15 is connected to a terminal 30 whichdelivers the filtered deviation measurement signal noted F_(k), which issuch that:

    F.sub.k =K.sub.k.ε.sub.k +(1-K.sub.k).F.sub.k.sub.-1

In certain operating conditions, for example in the presence of a targetwith very fast evolutions, it is necessary to limit the recording ofprevious deviation measurement values and hence their being recorded.

For this purpose it is necessary to add in the loop between the outputof the delay circuit 17 and the input of the multiplier circuit 16 amultiplier circuit 18, which is also connected to the output of a memory19 whose content is a predetermined positive constant number μ less than1, which enables the recording of previous measurements to be limited intime. In this case, signal F_(k) of the filtered deviation measurement,which is available on terminal 30, can be written:

    F.sub.k =K.sub.k.ε.sub.K +(1-K.sub.k).F.sub.k-1.u.

Under conditions of reception which may cause big fluctuations in thesignal, it is necessary to use deviation measurement receivers with anautomatic gain control. Circuit 27 is then modified. FIG. 2 shows areceiver with A.G.C. with circuit 27 modified in this particular case.This receiver contains a circuit 50, which is connected to the videooutput of the deviation measurement receiver, the automatic gain controlcircuit properly speaking receiving the sum Σ and difference Δ signalson two terminals 42 and 43 respectively. Automatic gain control circuit50 contains two variable gain amplifiers 40 and 41, a quadraticdetection circuit 38, a subtraction circuit 37, an amplifier 39 and anamplitude-phase detector 36. Modified circuit 27 includes an integratingcircuit 4 which contains, for example, a resistor 34 and a capacitor 35,a subtraction circuit 33 and a circuit 32 that calculates a functiong(y) in which y is the signal applied to its input, the choice offunction g being determined by the output signal of circuit 32 whichmust be equivalent to Σ² /Σ².

If G is the power gain of amplifiers 40 and 41, this signal coming fromamplifier 41 is equal to √G.Δ and that from amplifier 40 to √G.Σ.

Amplitude-phase detector 36 calculates the deviation measurement signalε from signals √G.Δ and √G.Σ coming from amplifiers 40 and 41. Quadraticdetection circuit 38 receives signal √G.Σ coming from amplifier 41 anddelivers at the input of subtraction circuit 37 the signal GΣ² ; this iscompared with a predetermined threshold V_(o), which is available atterminal 44 and fixes the amplitude of the signal that is required atthe output of circuit 50.

Signal (V_(o) -GΣ²) coming from subtraction circuit 37 is amplified byamplifier 39 whose output, which is connected to the gain controls ofamplifiers 40 and 41 and to the input of circuit 27, delivers a gaincontrol signal W, which is proportional to signal V_(o) -GΣ². To ensurethe stability of the gain loop with a big variation dynamic of signal Σ,the gain of amplifiers 40 and 41 must be an exponential function ofsignal W, i.e.: G=G_(o).10⁻αW in which G_(o) and α are predeterminedpositive numbers; when the gain loop is locked on, G≈V_(o) /2 or Σ²=(V_(o) /G_(o)).10.sup.αW.

Circuit 27, which receives signal W, calculates its mean value W bymeans of integration circuit 4.

Subtraction circuit 33 receives signals W and W and delivers to theinput of circuit 32 a signal W-W such that

    W-W=G(Σ.sup.2 -Σ.sup.2)

Circuit 32 delivers at its output a signal defined by the functiong(w-w)=10.sup.α(W-W) which, because of this, is equal to the signal U=Σ²/Σ² previously defined and which is applied together with deviationmeasurement signal ε to circuit 20 that delivers the filtered deviationmeasurement F_(k) at terminal 30.

A device for reducing the scintillation noise for electromagneticequipments making deviation measurements has thus been described.

What is claimed is:
 1. A device for reducing the angular scintillationof moving targets, which is used more especially in electromagneticequipment making deviation measurements and containing a receptioncircuit, which reception circuit delivers to terminals of said devicethe sum signal Σ, the difference signal Δ, the deviation measurementsignal ε calculated from said sum Σ and difference Δ signals, a signal Drepresenting the radar target range and a signal Vr representing theradial speed of the detected target, said device further comprising afirst circuit which receives the sum signal Σ and delivers signal Urepresenting the normalized fluctuations of the signal Σ² with regard toits mean value Σ² and a second circuit which receives signal U and thedeviation measurement signal ε and contains a peak detector circuitwhich delivers the signal V corresponding to the peak values of U, afirst means which is supplied with the output signal U of the firstcircuit and with the output signal V of the peak detector circuit anddelivers a signal x_(k) =U/V, a first calculating circuit connected atthe output x_(k) of the first means and calculating the signal K_(k)=f(x_(k)) where f is such a predetermined non-linear function that thesignal K_(k) tends rapidly towards 1 or 0 for a small variation of theinput signal x_(k), and a second means, supplied with the output signalK_(k) of the first calculating circuit and with the deviationmeasurement signal ε from the reception circuit, delivering the valuesF_(k) =K_(k) ε+u(1-K_(k))F_(k-1) of ε which correspond to the maximumvalues of the sum signal Σ, F_(k-1) being the preceding filtered valueof ε and u a predetermined positive number less or equal to
 1. 2. Adevice for reducing the angular scintillation according to claim 1,wherein the first circuit determining the ratio U comprises at least asquaring circuit which delivers from the sum signal Σ available on afirst terminal and signal Σ² to the first input of a divider and to anintegrator, the output of which integrator is applied to the secondinput of said divider so said divider supplies at its output the signal:

    U=Σ.sup.2 /Σ.sup.2


3. A device for reducing the angular scintillation according to claim 2,wherein the first circuit further comprises a first adding circuitconnected between the output of the integrator and the second input ofthe divider and a correcting circuit which receives the signal D from asecond terminal, the signal Σ² delivered by the integrator and thesignal Vr from a third terminal and delivers to the second input of saidfirst adding circuit a signal equal to -4Σhu 2.(v_(r) /D)τ, τrepresenting the delay time introduced by the integrator.
 4. A devicefor reducing the angular scintillation according to claim 1, wherein thefirst circuit comprises an integrator the input of which is suppliedwith a signal W and the output W of which is connected to a subtractingcircuit, each one of said integrator and subtractor receiving the signalW controlling the gain of first and second amplifiers in a receiverautomatic gain control circuit, said first and second amplifiersreceiving the difference Δ and sum Σ signals respectively, and a secondcalculating circuit receiving the output signal W-W of the subtractorand delivering signal U=10.sup.α(W-W), in which α is a predeterminedpositive number.
 5. A device for reducing the angular scintillationaccording to claim 1, wherein said second means comprises a firstmultiplier fed with the deviation measurement signal ε and with theoutput signal K_(k) of the first calculating circuit, a secondcalculating circuit fed with the output signal K_(k) of the firstcalculating circuit and delivering a signal (1-K_(k)) to the first inputof a second multiplier, the output of which is connected to the firstinput of an adding circuit receiving on its second input and outputsignal of the first multiplier and supplying its output signal to theinput of a delay circuit, the output of which is connected to the secondinput of the second multiplier through a third multiplier receiving on asecond input the contents μ of a memory, said adding circuit deliveringthe filtered deviation measurement signal F_(k).
 6. A device forreducing the angular scintillation according to claim 5, wherein saidfirst means comprises a divider circuit calculating the ratio x=U/V ofthe output signal U of the first circuit to the output signal V of thepeak detector circuit, and a first sampling circuit delivering thesignals x_(k) to a first digital circuit coding the amplitude of thesamples x_(k) and further including a second digital circuit coding theamplitude of the deviation measurement signal ε from a first terminalwhich are first sampled in a second sampling circuit, and fed to saidfirst input to produce signal K_(k) of said first multiplier, krepresenting the number of the sample.